X is a science fiction space trading and combat simulator series created by German developer Egosoft. The series is set in the X-Universe where several races populate a number of worlds connected by jumpgates.

Hundreds of millions of years ago, several races (nicknamed "the Ancients") transcended their physical form, combining into a single collective consciousness, and built billions of jumpgates throughout the universe. Their purpose was to limit the development of faster-than-light travel in other races, inducing them to rely on the gates to explore the universe. This enables the Ancients to act as shepherds of life, preventing wars between civilizations and monitoring the development of new ones.

However, the Ancient failed to notice the rise of humanity and never placed jumpgates in the Solar System, so they were blindsided when mankind discovered the principle behind wormholes by chance in 2022. Over the course of the 21st century, humans experimented with their own jumpgate technology and successfully built a pair linking Earth and Mars, paving the way for the colonization of the Solar System and beyond: one of the two jumpgates was launched towards Alpha Centauri in an attempt to claim a foothold in interstellar space, but in 2041, Earth lost contact with it, and while trying to re-establish the connection, instead locked onto Ancient-made ones, causing them to stumble into the X-Universe. In response, the Ancients rearranged some gates to contain humans in their own isolated network devoid of other life. In 2045, humanity began to explore this sub-network, finding plenty of suitable planets for terraforming and colonisation. Self-replicating AI ships, the Terraformers, were sent out into the network to find and terraform planets, but in 2145, ships corrupted by a final update meant to install a self-destruct protocol (which was both badly-made and sabotaged by the disgruntled head of the terraforming project) started targeting inhabited worlds, killing billions. Earth was saved by Nathan Ridley Gunne, the commander of its space navy, who lured the Terraformer fleet out of the Solar System before the human jumpgate was destroyed behind them. Earth was saved, but the stranded colonists discovered the alien gates now led to a completely different network.

This part of humanity now separated from Earth survived and formed a new civilisation called the Argon Federation, a name coming from a corruption of "R. Gunne". The Argon gradually lost their cultural connection with Earth, even adopting a new year counting system (NT, for New Time) in 2170 and starting the narrative that Earth never existed, partly to prevent the remaining Terraformers from finding a new way back to the Solar System. Those that resisted this decision kept archives of "Old Earth" in hope of an eventual reunion, eventually evolving into what was considered a fringe cult in Argon society, called the Goner. The new configuration of this network, while still limited to the Milky Way, did not leave humans alone however, and the Argon first encountered an alien species, the Paranid, in 130 NT. The Boron followed in 323 NT, the Split in 324 NT, and the Teladi in 550 NT. Over the centuries, attacks from Terraformers, now known as the Xenon, would occasionally flare up, eventually leading into a war called the Xenon Conflict in 225 NT. Due to another war, the Boron Campaign, breaking out between the first four races in the wake of the Split first contact, all five gradually formed the Community of Planets to try and maintain peace between themselves.

Around 730 NT (2912), during a new buildup of Xenon forces, a jumpship from Earth piloted by Kyle William Brennan suddenly appeared in the Teladi capital system Seizewell, kicking off the events of the games proper.

The X-Universe features several sentient species:

The series, which was launched in 1999 on the Windows platform, consists of five base games: X: Beyond the Frontier, X2: The Threat, X3: Reunion, X Rebirth and X4: Foundations. The series games are expanded by sequels that both add features and extend the plot. A plot can be undertaken in all games except X-Tension where the player takes part in missions to unfold events.

In 2010, Egosoft released the X-Superbox, a pack containing all the X series games up to that date: X: Beyond the Frontier, X-Tension, X2: The Threat, X3: Reunion, and X3: Terran Conflict. Included in the pack was extra missions within Terran Conflict. Additionally, owners of the pack also obtained X3: Albion Prelude for free when it was released in 2011.